The study, published today in Nature, updates the estimate for the total number and density of trees on Earth. It found that each year more than 15 billion trees are lost through a combination of deforestation, disturbances such as fire, and changes in land use. Since the beginning of human civilisation, the world has lost 46% of its trees.
The research team, led by Thomas Crowther, postdoctoral fellow at the Yale University School of Forestry and Environmental studies, challenges previous estimates from satellite data by using more than 400,000 on-ground tree density estimates to produce the first complete global map of trees.
Crowther said the map contributes to our fundamental understanding of the Earth’s ecosystems, allowing us to comprehend the global forest spread in terms of tree numbers.
There are 1.39 billion trees in tropical and subtropical forests, 610 billion in temperate regions, and 740 billion in northern, boreal regions.
Crowther argued that without a solid baseline of tree numbers it is difficult to develop targets and projects to conserve forests.
Bill Laurance, a biologist at James Cook University, said the findings of the study were both striking and alarming.
“It’s remarkable that there are so many trees on the planet but concerning that we are losing the trees so rapidly,” he said.
Laurance said it was important to consider the consequences of the shift in tree sizes, where small trees are rapidly replacing large trees that are vulnerable to environmental factors such as drought and human intervention through logging.
Rod Keenan, a forestry professor at University of Melbourne, said “high tree numbers may not always be a good indicator of forest condition”.
Lots of trees may indicate young forests recovering from disturbance, and tree numbers would decline as forests mature, he said.
The worst tree loss is already known to be in tropical regions but the global scale of forestry decline highlights how historical land use has shaped natural ecosystems.
Peter Kanowski, a forestry professor at Australian National University, said “all sorts of forests are valuable for all sorts of reasons, and we’re losing too much of them”.
“The scale and rate of loss of biodiversity, carbon stock and other forest values, that has been at unprecedented rates for much of the past 50 years, is impacting profoundly and perhaps irreversibly on ecosystems, livelihoods, forest values and climate at scales from global to local,” he said.
